The present invention pertains to screens used to form microwave cavities associated with electrodeless lamps. Examples of such screens are disclosed in PCT Publication Nos. WO 97/27617 and WO 97/27618; U.S. Pat. No. 4,673,846; and U.S. Pat. No. 5,397,966, each of which is incorporated by reference herein. Typically, the screen or mesh is made of electrically conductive material or metal, commonly stainless steel foil having a thickness of about 0.005 inches or less. The mesh size is typically selected to optimize various factors including RF shielding, light transmission, electrical conductivity, and thickness for structural strength.
The mesh material may be formed from overlapping wires, as shown in FIG. 1, or may be etched, as shown in FIG. 2. An etched mesh has the advantage of lower contact resistance at the mesh intersections. Also, a mesh of overlapping wires may be plated, for example, with an electrically conductive metal, such as stainless steel, so that the mesh intersections become integrally jointed.
Typically, the mesh is formed to provide a screen portion of a microwave cavity. For example, FIG. 3 shows an electrodeless lamp which includes a magnetron 1 which generates microwave energy. The microwave energy is radiated by an antenna 2 into a waveguide 3. The waveguide 3 directs the microwave energy through an opening 5 to a cavity, defined by a reflector 4 and a screen 9, where the microwave energy excites the material inside a bulb 6. In FIG. 3, the screen 9 is flat and has a circular shape with a diameter corresponding to the diameter of the opening of the reflector 4. Other flat screens may be formed with any number of possible shapes. For example, FIG. 4 shows a screen 19 which is flat and has a square shape which corresponds to a square opening of a reflector 14 which houses a bulb 16.
In FIGS. 3 and 4, the walls of the respective reflectors 4, 14 substantially define the microwave cavity and the corresponding screens 9, 19 have a two dimensional (i.e. flat) shape. Other configurations are also possible where the screen has a three-dimensional shape and the screen itself substantially defines the microwave cavity For example, FIG. 5 shows a configuration where the screen 29 surrounds the bulb 26 and the reflector 24 is separate from the cavity defined by the screen 29. In another example, as shown in FIG. 6, the screen 39 may be box-shaped with walls 37 and a top 38. Other three-dimensional shapes are also possible.
A problem with prior art screens is that the screen material degrades during operation. For example, various screen materials may oxidize or tarnish over the life of the lamp. The degradation causes various undesirable effects including, for example, reduced light output.
The present invention is directed to a screen for an electrodeless lamp which includes a coating on the screen. The screen includes a conductive mesh which bears a protective coating for inhibiting degradation of the screen. For example, the coating protects the screen from oxidizing and/or tarnishing. An exemplary coating may comprise a glass coating with a controlled set of material properties to provide the above-described protection under lamp operating conditions for at least about 1000 hours and without substantial cracking as the screen heats and cools.
Preferably, the coating includes a diffusion barrier to substantially reduce the diffusion of oxygen and/or sulfur onto the screen material. More preferably, the diffusion barrier also substantially reduces the sublimation of the screen material. The diffusion barrier includes, for example, one or more of the following materials: silica, single phase glass, two phase glass, and aluminum. The diffusion barrier is effective to inhibit degradation and the diffusion barrier does not substantially crack at a screen temperature of greater than about 300xc2x0 C.
The protection provided by the coating extends the useful life of the screen and, in some circumstances, the reflector. The coating may be substantially transparent or diffusely reflective or specularly reflective. The coating may be applied, for example, through a sol-gel process. For example, the coating may be applied by preparing a solution which includes an organic precursor of silica, applying the solution to at least a portion of the screen, drying the applied solution, thereby leaving a deposit on the screen which includes silica, and firing the screen at a temperature sufficient to convert the deposit to a glass. The applying step includes spraying the solution on the screen or dipping the screen in the solution and withdrawing the screen, preferably at a uniform rate, the rate being determined to provide a desired thickness of the coating.
The step of preparing the solution includes, for example, mixing a first solution of magnesium nitrate (3% to 16% by weight), aluminum nitrate (3%to 8% by weight), MeOH (16% to 28% by weight), and TEOS (7% to 24% by weight) with a second solution of MEK (35% to 40% by weight), phosphoric acid (4% to 10% by weight), and MeOH (2% to 4% by weight).